FIG. 1 illustrates a schematic view of an essential part of an image forming apparatus of an embodiment according to the present invention. As shown in FIG. 1, the image forming apparatus comprises a heating source 1, a recording roller 2, an inking unit 3, an intermediate transfer roller 4, a recording paper 5, a pressure roller 6, an infrared heater 7, a cleaning unit 8, a liquid layer-forming roller 9 and an image recording body 10. Examples of the heating source 1 (preferably light source having a wavelength from 300 nm to 1300 nm) include a light-emitting diode array (which is hereinafter referred to as LED array), a semiconductor laser, a solid laser or the like. The image recording body 10 has surface characteristics that a receding contact angle decreases when a surface of the image recording body in a heated state is brought into contact with a liquid (hereinafter referred to as a liquid-attracting state), and the receding contact angle increases when the image recording body is heated in a state where the image recording body is not contact with the liquid (hereinafter referred to as a liquid-repelling state).
When the surface of the image recording body 10 is treated to become the liquid-attracting state, a liquid layer is formed on the surface of the image recording body 10 before the surface of the image recording body 10 is heated, or a liquid is come into contact with the surface of the image recording body 10 during a heating operation, or the liquid is come into contact with the surface of the image recording body 10 right after the surface is heated. As shown in FIG. 1, it is possible to form the liquid layer on the image recording body 10 by means of the liquid layer-forming roller 9. However, it is not necessary to use the liquid layer-forming roller 9 for formation of the liquid layer. For example, it is possible for the image recording body 10 to remove from the recording roller 2 or for the recording roller 2 to dip in the liquid, or for the liquid layer to form on the surface of the image recording body 10 by means of the cleaning unit 8.
As mentioned above, after the surface of the recording body becomes the liquid-attracting state, writing of an image can be carried out thereon. This image writing can be performed by scanning the heating source 1 with separation of the image recording body 10 from the inking unit 3, the intermediate transfer roller 4, the cleaning unit 8 or the like. As a scanning method, writing can be carried out by moving the heating source 1 in a main scanning direction by a linear motor or the like while rotating the recording roller 2, or a raster scanning can be applied for writing by means of polygon mirror or galvano mirror.
The image is then formed on the image recording body 10 and at this image forming period the recording paper 5 is fed while pressing the image recording body 10 with the intermediate transfer roller 4 and the inking unit 3. As the inking unit 3, a plurality of inking rollers can be used in order to control a thickness of an ink layer by means of a blade or the like 3b. An ink can be supplied from an upper portion of the blade 3b by dropping an ink from an ink supply container 3a. 
After printing a desired press run, it is possible to reuse the image recording body 10 by forming another image thereon newly. At this time, that is to say, at the end of printing, the intermediate transfer roller 4 and the inking roller 3 are separated from the image recording body 10 and the cleaning unit 8 is brought into contact with the image recording body 10 so that the ink remaining thereon can be removed. After removal of the ink remaining on the image recording body 10, another liquid layer is formed on the surface of the image recording body 10 and the image recording body 10 is heated by means of the infrared heater 7 to treat the surface of the image recording body 10 so as to become the liquid-attracting state, while erasing the latent image of the previous image. However, in addition to the above method, as a method for treat the surface of the image recording body 10 so as to become the liquid-attracting state, it is possible to heat in a state where the liquid is come into contact with the surface of the image recording body 10 or for the surface of the image recording body 10 to bring into contact with the liquid right after heating the surface. In other words, as mentioned above, after printing a predetermined press run on the recording paper 5, the ink remaining on the image recording body 10 is removed and the latent image (a liquid-repelling region) formed on the image recording body 10 is erased, and the surface of the image recording body is then treated so as to become the liquid-attracting state. In this way, a new image (a latent image) can be formed on the image recording body 10 so that the image recording body 10 can be reused. That is to say, reuse of the image recording body 10 can be achieved by the following steps of: removing the ink remaining on the image recording body 10; forming the liquid layer on the surface of the image recording body by means of the liquid layer-forming roller 9; and heating the surface of the image recording body 10 by means of the infrared heater 7 to perform the liquid attracting treatment of the image recording body 10. As the cleaning unit 8, it is possible to use a waste 8b soaked with a cleaning liquid from the cleaning supply container 8a. The surface of the image recording body 10 is washed by pressing it with this waste 8b. 
In writing of the image into the image recording body, there are positive writing and negative writing. As negative writing, by heating the image recording body 10 in a state where the body 10 is come into contact with an element selected from the liquid and/or a solid, or brought into contact with the element selected from the liquid and/or the solid right after the surface of the image recording body 10 is heated, the receding contact angle of an image region of the image recording body decreases so as to subject the surface of the image recording body 10 to the liquid-attracting treatment. By selectively heating a non-image region of the image recording body in the absence of the above contacting element, the receding contact angle of the non-image region of the image recording body increases so as to subject the surface of the image recording body to the liquid-repelling treatment. On the other hand, as positive writing, by selectively heating only the image region of the image recording body 10 in a state where the body is come into contact with the element selected from the liquid and/or the solid, or selectively brought into contact with the element selected from the liquid and/or the solid right after selectively heating the surface of the body, the image region of the image recording body 10 is subjected to the liquid-attracting treatment.
As stated above, in the image forming apparatus according to the present invention, the liquid-attracting treatment is performed on the image recording body 10 according to image information. The ink is attached to portions of the image recording body 10 subjected to the liquid-attracting treatment and the ink attached to the body 10 can be directly transferred to the recording paper 5 through the intermediate transfer roller 4 so as to achieve transfer and printing steps.
As described above, the image recording body 10 can be reused by the following steps of: printing the predetermined press run onto the recording paper 5; erasing the latent image (the liquid-attracting region) formed on the body 10 and removing the ink-remaining thereon; and forming a new image (the latent image) on the body 10. That is to say, reuse of the image recording body 10 can be carried out by removing the ink remaining on the body 10 by use of the cleaning unit 8 or other means and then heating the image recording body 10 by means of the infrared heater 7 to provide the liquid-attracting state for the surface of the image recording body 10.
Japanese Laid-Open Patent Application No. 3-178478 describes a recording method by use of a thermosensitive material having changeable character of wettability. Japanese Laid-Open Patent Application No. 8-276663 describes a recording method by negative writing. However, the invention of the above patent application No. 3-178478 mainly relates to the recording method by means of a thermal head in which there may be occasions that a recording layer is mechanically damaged due to contact recording between the thermal head and a recording body having the recording layer. Also, for the recording method by negative writing described in the above patent application No. 8-276663, an efficiency of light-heat conversion of a light source is not enough high because of continuous oscillation of the light source. There are problems that writing for the above method is time-consuming and is required to employ an expensive device as the light source.
Further, in the recording method disclosed in the above patent application No. 8-276663, there are disadvantages that a lifetime of the light source is shorter as compared to positive writing.
Moreover, in a case where the above recording body having a light-heat converting layer, for example in a case of writing by means of laser light, when incident light from a surface of the recording layer is reflected at a surface of a substrate through the recording layer and the light-heat converting layer, regular reflecting component is strong and this component reaches a boundary surface between the light-heat converting layer and the recording layer or the surface of the recording layer, thereby leading to multi-reflections within the above layers. This results in spreading of exposed areas and a recording irregularity, such as a more.
In addition, in order to reuse the recording body used once, a clear printing can not be performed with a recording paper fouled by the remaining ink unless the ink remaining on the recording body is removed completely.
Furthermore, in a case of writing to the recording body by use of a plurality of heating means, if only one of the heating means is broken, a non-image region which is not heated is produced, thereby giving rise to a line of scumming.
In a case where negative writing is also carried out, an irregularity in heating and cumulative heat of an image forming means during negative writing operation cause the image to be degraded.
In a case where negative writing is also performed through a rotating multi-face mirror, if sensitivity of the recording body is low, a rotation rate of the rotating multi-face mirror must be low, so that the rotation rate is unstable because of the absence of an action of inertia induced by the rotation. This result in an irregularity of the light-heat conversion, thus producing irregularities relating to an image density, dot size and dot line.